1. Field of the Invention
This invention pertains to filter apparatus and more particularly to rotating filter apparatus for separating fine particles of solids from a liquid.
2. Description of the Prior Art
Filters traditionally employ a filter medium through which a fluid having solid particulates suspended therein is passed, leaving the particulates filtered out and permitting the filtered fluids to flow forward. Alternatively to a flowing fluid, filters can be passed through the fluid to remove the filtered particles by a screening operation.
Using either of such methods, it has long been believed that in order to filter out smaller particles, it was necessary to have a filter with smaller openings that blocked out the filtered particles. Hence, as filter mediums improved from screens to other mediums, sintered discs evolved as a preferred medium for filtering out particles of relatively small size, on the order of 75 microns. Such sintered discs comprise a combination of metallic plastic and/or ceramic materials that are held together by compression and/or adhesive or cementitious binding materials. The discs are porous, the opening therethrough for a predetermined thickness being determinable. Hence, down to a certain size of filtered particle it is possible to merely select the disc of proper size and constituent makeup.
However, as needs have arisen for filters having the capability of filtering particles of even smaller size, the limits of the filtering art using the technique of selecting a finer porous medium have been severely tested. The main reason for reaching this limit is the drastic reduction in flow-through rate caused by filters having smaller and smaller filter openings.
It was discovered, however, that it was possible to filter smaller particles than represented by the openings in a sintered disc by rotating the disc as the fluid passed through them. This is believed to be caused by a phenomena characterized as creating a "boundary" layer that builds up on such a rotating disc when the disc is rotated in a fluid.
A stream of fluid moving in a predominant direction at a speed less than that which causes turbulence to occur is believed to flow in established layers. When flowing in a confined channel, the layers next to the walls of the chamber do not flow as fast as the layers flowing in the center of the stream because of the surface tension of the channel walls with the adjacent layer. The layer next to a wall moves the slowest of the layers, the center layer moves the fastest and the layers between move progressively faster within these two extremes as they are established between the layer next to the channel wall and the center layer. The layer or layers next to the channel wall form the "boundary".
When a filter disc is rotated in a fluid, the relative rotation of the disc in the fluid creates a boundary of the type described. Actually, the boundary layer next to the surface of the disc moves faster, than the fluid at some distance therefrom, but it is closer in speed to the wall than further away and, hence, the creation of the boundary. The rotation or the spinning of the disc does cause the fluid to circulate because of the respective surface tension attraction of the fluid layers, but the layer closest to being the same speed of the disc is obviously the adjacent layer thereto.
When a particle moving in the fluid angles toward the porous surface of the disc it would be expected to pass through if such particle was of a size less than that of the opening. However, it has been observed that in the presence of an established boundary layer, particles of about 50% less in size than the opening are repulsed. That is, such particles bounce off the layer. In time, all particles that do not pass through the disc filter progress out by centrifugal force and once free of circulatory motion, descend through the fluid beyond the edge of the disc to the bottom of the fluid chamber.
When the rotation of the disc is too slow, then the boundary layer or layers becomes too thick and it is not a layer at all. This fluid acts as substantially still fluid. In such case, the boundary rejection phenomena just described is not at work. A round disc rotates at different surface speeds of rotation at different radii from the center. The speed is relatively slow near its center and is relatively fast near its periphery. Hence, when the speed of rotation is too slow to establish a boundary, the boundary disappears first toward the center. As the disc slows down, it loses its boundary further and further out from the center until the boundary finally disappears at the periphery.
Although possibly not appreciating the preferred speed of rotation to accomplish filtering utilizing the boundary layer phenomena, Ernest J. Breton did discover that there was a filtering advantage of rotating discs, as revealed in U.S. Pat. No. 3,997,447, issued Dec. 14, 1976 and entitled "Fluid Processing Apparatus". In the system described in his patent, a rotatable hollow shaft is placed into the fluid to be filtered, the shaft having attached to it a rotatable manifold to draw the filtered fluid off and stacked rotatable sintered discs secured to the hollow shaft. The shaft is open to both outlet manifold and to the filtering discs. A pressure gradient is created by a suction at the output. Hence, the filtered particles are removed by rotating discs. The fluid is back pulsed periodically to remove the stuck particles which have plugged the surfaces of the discs.
It should be noted that Breton does not reveal a mechanism by which fluid is introduced to the vessel in which the Breton apparatus operates. The removal of the filtered particles is not specified. The filter disc assemblies are securely attached to the surface of the hollow shaft at the point of the discs. The pressure differential is established by a suction at the fluid outlet. Back pulsing is periodic.
Although the Breton apparatus included many suitable features, it is abundantly demonstrable that the long existence of the publication of the Breton patent without commercialization indicates that improvements needed discovery to make the concept commercially acceptable.
Therefore, it is a feature of the present invention to provide an improved rotary filter apparatus in which the fluid to be filtered is introduced into the chamber housing tangentially to the rotating mechanism to strikingly provide an improved particle separation result.
It is another feature of the present invention to provide an improved rotating filter apparatus in which the particles separated in the filtering process are removed from the chamber without interfering with filter operation.
It is still another feature of the present invention to provide an improved back pulsing system for operation with a rotating filter that does not interfere with the basic -filtering operation.
It is yet another feature of the present invention to provide an improved procedure of establishing a pressure differential across a rotating filter compatible with solid particles removal as well as clean fluid removal.